This invention relates generally to digital electronic imaging systems. More specifically, the present invention relates to an apparatus and method for efficiently choosing a thresholding algorithm to be applied to regions of an image represented by optical reflectivity or transmissivity signals at each image element (pixel).
In the typical digital imaging system a specimen (or document) is illuminated and then scanned at every pixel to obtain analog reflectivity values. The analog reflectivity values are converted to a multi-digital number by an analog-to-digital converter, where the multi-digital number ranges from a minimum value representing the darkest state possible for the pixel, decimal number 0 for example, to a maximum value representing the lightest state possible for the pixel, decimal number 63 for example.
The multi-digit number representing the reflectivity of the pixel is compared to a threshold value (sometimes dynamic), so that the final digitized picture is represented by a single binary digit for each pixel. Stated differently, each pixel in the final digitized picture is stored or displayed in either an "ON" state or an "OFF" state. In comparing the multi-digit number to the threshold value the resulting pixel value is obtained as follows: if the multi-digit value exceeds the threshold value the pixel is determined to be in the "ON" state; if the multi-digit value equals or is less that the threshold value the pixel is determined to be in the "OFF" state.
It has been observed that the optical characteristics of digital images vary with predictability depending on the type of specimen. For example, photographic (or grey scale) specimens noticeably differ from text or graphic specimens in the their pixel patterns. Also, it has been observed that some thresholding methods operate more efficiently in digitizing one type of specimen, while being less efficient in digitizing another type of specimen. For example, a particular threshold method might provide a very accurate representation of a text specimen, while providing a poor representation of a photo specimen.